The invention provides a control circuit for light emitting diode arrangements, having at least two circuit branches for at least one light emitting diode arrangement and having a balancing device for balancing the currents in the individual circuit branches.
These kinds of control circuits find application, for example, in modern flat-screen televisions having an LCD display in order to control the required backlighting. At the edges of the screen, for example, a plurality of light emitting diode arrangements having light emitting diode strings for example, are thereby disposed, which, using suitable diffusion disks, illuminate the entire screen. Here, the light emitting diode arrangements are separately controlled in separate control channels so as to keep the required voltages low.
An important aspect of high-quality in these kinds of televisions is uniform illumination over the entire surface of the screen. That is why the control circuit has a balancing device, such as a transformer arrangement, to balance the currents in the individual control channels. In this way, the current that flows through each light emitting diode is balanced, thus making the brightness of all light emitting diodes mostly identical as well.
In the prior art, various transformer arrangements of this kind are known and the person skilled in the art will be able to choose a suitable arrangement from this selection. A large number of active balancing circuits that do not need a transformer are also known from the prior art, these, however, are more complex and more expensive.
Furthermore, it is normal for the brightness of the backlight to be adjustable, making the image adaptable to different environments. For this purpose, the control circuit has a dimming device allowing the overall brightness to be changed. This is usually effected through a change in voltage on the primary side of the transformer arrangement.
This kind of control circuit is known, for example, from US 2010/0237799 A1 and is shown in FIG. 1 by way of example. The control circuit has a mains connection 26, an input filter 24, a power factor correction circuit 2 and an inverter 3. In the example, the circuit has N control channels 5 of which three are illustrated. A balancing transformer 8 is disposed in each control channel 5, the primary windings 12 being connected in series to the inverter 3. On the secondary side of the transformers 8, each control channel 5 has a secondary winding 7, a bridge rectifier 9, smoothing capacitors Cp 10 and a light emitting diode arrangement 11 each having two light emitting diodes 25 that are connected in a well-known manner.
If, in the illustrated circuit, a light emitting diode or a light emitting diode string should fail due to a defect, the current in this control channel drops to zero. As a result, the balancing of currents is impaired, which is why the entire circuit can no longer be operated. This is also why it is only possible to dim all the control channels jointly over the primary side, otherwise currents of varying intensity would also flow in the individual control channels.
Moreover, the control circuit has long current rise and current fall times (>50 μs) during dimming, which is due to the leakage inductance of the transformers and the capacitance of the smoothing capacitors.